The invention relates to a method of calibrating an equilibrium position of an electrically operated actuator in a motor vehicle clutch or in a gear-selecting mechanism, where the electric motor driving the actuator is assisted by a compensating spring.
Electrically operated actuators of the kind that the present invention relates to are disclosed for example in GB 2325036, GB 2313885, or GB 2309761. The disclosures of these documents are expressly incorporated herein by reference. Such actuators include an electric motor, which drives a hydraulic master cylinder that communicates with a slave cylinder which, in turn, actuates a vehicle clutch or a gear-ratio selecting mechanism. The electric motor in these actuators can work through an appropriate gear mechanism such as a worm-drive mechanism to drive a push rod. One end of the push rod is connected to a crank that is tied to the gear wheel of the worm-drive mechanism, while the other end of the push rod is connected to a piston that slides in a master cylinder, so that the rotary movement of the gear wheel is converted into a linear movement of the piston. The electric motor, the gear mechanism, and the master cylinder are preferably arranged together in a common housing.
The master cylinder of the electric actuator described above is typically connected to a slave cylinder of a clutch. When pressure is applied to the slave cylinder, a clutch release fork is actuated which acts on a clutch release bearing to generate a force that disengages the clutch. The release bearing typically acts on a diaphragm spring which in its normal (i.e., non-actuated) state holds the discs of the clutch in frictional engagement. When the diaphragm spring is depressed by the release bearing, the clutch discs move apart, so that the clutch becomes disengaged. The force generated by the electric motor therefore has to be large enough to depress the diaphragm spring to an extent that is sufficient to release the engagement of the clutch. The force required to disengage the clutch is typically of the order of 450 N.
In order to reduce the size of the electric motor required for such actuators, it has been proposed to include a compensating spring in the electric actuator, so that the compensating spring counteracts the opposing force of the diaphragm spring. This may be accomplished for example with an arrangement where the compensating spring is fully compressed in the completely engaged state of the clutch, whereby-the compensating spring generates a force of, e.g., 250 N in the electric actuator in the direction of disengagement of the clutch. In the process of disengaging the clutch, the initial amount of force to depress the diaphragm spring will now be supplied by the compensating spring. Although the force of the compensating spring decreases over the phase where the compensating spring and the diaphragm spring counteract each other, the electric motor only needs to generate a force of the order of 200 N to fully disengage the clutch. Thus, the requirement for the electric motor to produce 450 to 500 N, the amount of force that would be required without the compensating spring, can be reduced to 250 to 300 N through the use of a compensating spring.
In electric actuators of the type disclosed in the aforementioned references, a high level of static friction between the worm and the gear wheel provides a self-holding effect. However, in the interest of optimizing the efficiency of the actuator, it may be desirable if the internal static friction of the actuator is smaller than would be required to keep the actuator immobilized. In this case, it is possible that the force exerted by the diaphragm spring in the disengaged state of the clutch will force the actuator back, or that the force exerted by the compensating spring in the engaged state of the clutch will push the actuator forward, i.e., in the direction of disengagement. If this causes the actual position of the actuator to deviate from the required position by more than a predetermined tolerance, the controller will reactivate the actuator motor.
The German Patent Application DE 10062456.1, which is hereby incorporated by reference in the present disclosure, proposes the following concept to counteract the force of the diaphragm spring which could cause an unintended re-engagement of the clutch: When the actuator is in its rest position, a voltage of typically 7 percent of the maximum PWM voltage (Pulse Width Modulation voltage) is applied to the electric motor in the direction where the motor will support the compensating spring to counteract the force of the diaphragm spring. This voltage generates a force in the actuator which prevents the actuator from moving backward because of the reactive force of the diaphragm spring. However, in order to prevent a forward movement of the actuator in the engaged state of the clutch, the 7 percent voltage is applied only when the force generated by the compensating spring is smaller than the force produced by the diaphragm spring.
Consequently, this system requires that the equilibrium position is known where the forces of the compensating spring and the diaphragm spring keep each other in balance. In clutches with self-adjusters that adjust the position of the pressure plate to compensate for the wear on the friction surfaces, the equilibrium position remains substantially unchanged over the life of the clutch. In clutches of this type, the equilibrium position may be precalibrated. However, in a clutch without the self-adjusting feature, the equilibrium position changes significantly as the components of the clutch wear down in use. In clutches of this latter type, the concept of energizing the electric motor with 7 percent of the maximum PWM voltage cannot be used, and a much more complex adaptive strategy with a continuous current supply has to be used.
The present invention therefore has the objective to provide a method of calibrating the equilibrium position where the forces of the diaphragm spring and the compensating spring counterbalance each other, so that an initial calibration can be performed at the end of the assembly line after the system has been installed in the vehicle, and recalibrations can be performed at regular time intervals, for example with the routine maintenance services, so that the concept of using the assistance of the electric motor at 7 percent of the maximum PWM voltage can be used in clutches without self-adjusters.
A method according to the present invention serves to calibrate the equilibrium position of a clutch actuator that is driven by an electric motor and assisted by a compensating spring, where the latter is used to compensate an opposing elastic force of the mechanism that is operated by the actuator. The method includes the steps of:
applying a large-amplitude, high-frequency alternating position signal to energize the electric motor, where the position signal alternates between extremes that correspond to actuator positions spanning across the equilibrium position,
measuring the actual position of the actuator by means of a position sensor associated with the actuator, and
determining the equilibrium position as the position into which the actuator settles as long as the alternating position is applied.
Due to the combined force/displacement characteristic of the compensating spring and the counteracting elastic force of the mechanism, the actuator with the assistance of the compensating spring moves rapidly into the equilibrium position, while the speed of the actuator movement is significantly reduced when the motor is working against the opposing elastic force of the mechanism. Likewise, when the motor is energized in the reverse direction, the actuator returns rapidly to the equilibrium position and then continues slowly beyond the equilibrium position as the compensating spring is being compressed. As a result, the range of the movement of the actuator is centered on the equilibrium position. The higher the frequency of the alternating signal, the shorter the distance by which the actuator moves in either direction beyond the equilibrium position. Consequently, the equilibrium position can be determined more precisely by using a higher frequency. According to a preferred embodiment, the frequency of the alternating position signal is 25 Hz or higher. Particularly preferred is a position signal with a frequency of about 50 Hz.
The closer the midpoint of the alternating position signal is to the equilibrium position, the more accurate will be the result of the equilibrium determination. Consequently, it is possible to use an iterative technique of successive determinations of the equilibrium position where in each iteration the midpoint of the alternating position signal is positioned on the equilibrium position determined in the previous step until the equilibrium position coincides with the midpoint of the alternating position signal. Initially, the midpoint of the alternating position signal may be set so that it coincides with a theoretical equilibrium position calculated from the design characteristics of the actuator and mechanism or with the last known equilibrium position at which the system of actuator and mechanism was recalibrated.